The separation of hydrogen from gas mixtures is a crucial process in existing and envisioned uses of hydrogen as a chemical feedstock and fuel. Membranes comprised of thin films of metals are a well-known technology for achieving the separation. The membranes are designed with multiple performance objectives in mind, such as delivering high hydrogen flux, showing long-term operability over broad ranges of temperature and pressure, and resistance to poisoning and degradation by gas contaminants.
In separation operations, hydrogen permeates through metal membranes by means of a unique mechanism in which H2 dissociatively adsorbs on the catalytically active surface of the metal, producing hydrogen atoms which diffuse through the interstices of the bulk metal lattice and recombine on the opposite surface. Diffusion through the bulk metal is limited to hydrogen atoms, and an infinite selectivity for H2 separation can be achieved. Generally, for sufficiently thick membranes with sufficiently high H2 dissociation rates on the surface, hydrogen atom diffusion through the bulk limits the overall rate of hydrogen transport. Palladium (Pd) has been distinguished from other pure metal membranes by the high H2 dissociation activity of the Pd surface and the high hydrogen atom permeability of bulk Pd. However, in pure Pd membranes, alpha beta phase transformations during hydrogen loading/un-loading and temperature cycles introduces severe lattice strains, causing pure Pd membranes to become brittle. This can be greatly mitigated by alloying Pd with other metals, such as Silver (Ag), Copper (Cu), Gold (Au), Yttrium (Y), Cerium (Ce) and others.
Pd—Cu alloys are particularly effective due to a high H2 diffusivity and a resistance to sulfur compounds such as H25 which may also be present in a gaseous mixture. Additionally, Pd—Cu alloys exhibit B2 and fcc phases over the range of composition and temperature. See, e.g., Subramanian et al., “Cu—Pd (Copper-Palladium),” Journal of Phase Equilibria, Vol 12, No. 2 (1991). It is generally accepted that the B2 phase fosters a higher permeability value than the fcc phase due to a higher mobility of hydrogen atoms within the B2 crystal structure. See e.g., Howard et al., “Hydrogen permeance of palladium-copper alloy membranes over a wide range of temperatures and pressures,” Journal of Membrane Science 241 (2004); see also Opalka et al., “Hydrogen interactions with the PdCu ordered B2 alloy,” Journal of Alloys and Compounds 446-447 (2007). However, implementation of Pd—Cu alloy membranes is challenging due to Pd—Cu phase behavior and the nature of the B24fcc phase transition. A single B2 phase composition may not be stable upon thermal cycling to higher temperatures, and the formation of the fcc phase with a different molar volume compromises the integrity and H-selectivity of the membrane in such operations. Further, persistent structural and stoichiometric segregation may be induced by cycling through the two phase region (B2+fcc), and residual fcc phases can reduce the H2 permeability of B2 membranes significantly. Another aspect of fcc/B2 mixed phase morphologies is a possible propensity for defect formation along the grain boundaries between fcc and B2 domains. See e.g., Goldbach et al., “Impact of the fcc/bcc phase transition on the homogeneity and behavior of PdCu membranes,” Separation and Purification Technology 73 (2010). Correspondingly, increased stability of the B2 phase over a wider temperature range would be an advantageous feature of a Pd—Cu based membrane, and would provide significant utility for applications that require frequent thermal cycling, such as cycling down to ambient temperatures.
Further, stability over a wide temperature range would preserve the higher permeability value of the B2 phase of Pd—Cu based membranes during operations at temperatures above the Pd—Cu critical temperature. The maximum temperature at which any bcc phase is stable is about 873K (600° C.). As a result, in certain operations such as coal gasification, steam reforming, and catalytic partial oxidation, where temperatures of up to 900° C. may be expected, maintenance of the B2 phase and the resulting higher H2 permeability is not realizable. It would be clearly advantageous to provide a Pd—Cu based alloy for H2 separation membranes capable of maintaining a B2 phase and the associated H2 permeability values for these higher temperature operations. Another specific advantage could accrue in potentially enabling membrane reactors to simultaneously employ H2 and O2 membranes during autothermal reforming, where endothermic heat requirements are met by partially combusting or oxidizing methane. Currently, oxygen membranes based on dense mixed ionic electronic conducting perovskite materials are operated above 950-1000° C., which clearly exceeds the B2 phase critical temperature of generally about 600° C. for Pd—Cu H2 separation membranes. This dichotomy has necessitated purposely created temperature zones for the respective H2 and O2 membranes rather than integration of the membranes into one single unit. See e.g., Patil et al., “Design of a Novel Autothermal Membrane-Assisted Fluidized-Bed Reactor for the Production of Ultrapure Hydrogen from Methane,” Ind. Eng. Chem. Res. 44 (2005), among others. A Pd—Cu based alloy exhibiting B2 phase stability over a wider temperature range would significantly mitigate the operating temperature ranges and potentially allow for a significantly more streamlined operation.
Ternary alloys based on Cu—Pd based compositions have been investigated for use as H2 separation membranes using first-principle methods on high Pd content (>70 at. %) ternary alloys of Cu, Pd, and a metal M. These computational efforts concentrate on Pd rich compositions where the materials exist as substitutionally random fcc alloys, in order to preserve the higher H2S resistance observed in fcc structures. Ternary Cu—Pd-M alloys in an fcc structure, where Pd is present at 70 at. % or greater, and where M=V, Zr, Nb, Ru, Rh, Ta, Ti, Pt, Ag, and Au, have been investigated. See e.g., Kamakoti et al., “Towards first principles-based identification of ternary alloys for hydrogen purification membranes,” Journal of Membrane Science 279 (2006); see also Coulter et al., “Predicting, Fabricating, and Permeability Testing of Free-Standing Ternary Palladium-Copper-Gold Membranes for Hydrogen Separation,” J. Phys. Chem. C 114 (2010); see also Semidey-Flecha et al., “Detailed first-principles models of hydrogen permeation through PdCu-based ternary alloys,” Journal of Membrane Science 362 (2010); see also Ling et al., “First-principles screening of PdCuAg ternary alloys as H2 purification membranes,” Journal of Membrane Science 371 (2011). These efforts limit the ternary compositions to compositions providing Pd-rich fcc regimes, and do not investigate or discuss compositions leading to potential expansion of applicable temperature ranges for the B2 phase of ternary Cu—Pd-M alloys.
It would be advantageous to provide an H2 separation membrane comprised of a ternary Cu—Pd-M alloy which exhibits and extends a B2 phase, and which mitigates the B24fcc transition in cycling temperature operations. Such a membrane would mitigate negative impacts of the B24fcc transition on mechanical stability, such as changes in molar volume and defect formation along B2 and fcc boundaries. Further, such a membrane would mitigate the impact of persistant structural and stoichiometric segregation induced by repeated cycling through a two phase region (B2+fcc). Extension of the B2 phase would additionally preserve higher permeability values during operations at temperatures above the Pd—Cu critical temperature.